The present invention relates generally to use of approximate zero crossings for frequency determination.
In EW applications, it is desirable to measure a signal frequency very precisely. However, the measured frequency accuracy is limited by the measurement time or pulse width. An ideal approach is to measure frequency as a function of time. The longer the pulse width is, the finer the frequency can be measured.
Frequency can be measured using phase measurement. One scheme is to use a two-channel (I and Q) down converter, a phase digitizer and a phase encoder. Data collected in this manner is complex. The frequency can be measured very accurately, if the pulse width is long.
It is known that one can measure frequency by using zero crossings. However, the accuracy is limited by the clock cycle.
The following United States patents are of interest.
U.S. Pat. No. 4,982,165 - Lowenschuss
U.S. Pat. No. 4,644,268 - Malka et al
The Lowenschuss patent for a "Set-On Oscillator" to control the frequency of a jammer in an electronic warfare system, discloses a receiver having zero crossing detector means coupled to a frequency counter to determine the frequency of a received radar signal.
The Malka et al patent is for "Apparatus and Method For Determining the Magnitude and Phase of the Fundamental Component of a Complex Waveshape". So that the exact timing of the complex waveshape relative to a reference signal is established, the reference signal is applied to a zero crossing detector. The zero crossing of the reference signal drives a phase-lock loop and control logic to initiate a first digitized sample of a measurement cycle.